A conductor is obtained by assembling together a large number of such strands. A strand is itself constituted by assembling a very large number of superconducting filaments which are separated from one another by a non-superconducting separation material.
It is known that superconducting filaments, in particular those of niobium-titanium, are capable of transporting very high current densities of about 10,000 A/mm.sup.2, and that conductors based on such filaments are capable of having very low AC losses, under the following conditions:
1) The filaments must be ultrafine, having a diameter of less than 0.2 .mu.m, and they must be continuous, i.e. of uniform section and without interruptions.
2) The filaments must be separated by a material capable of reducing proximity effects sufficiently which would otherwise give rise to supercurrents flowing between filaments.
3) The material separating the filaments should be highly resistive so as to limit induced current losses.
4) The filaments must be finely transposed.
A presently known way of making a multifilament superconducting strand starts from an initial billet constituted by a core made of an alloy of niobium and titanium disposed inside a matrix made of an alloy of copper and nickel, with a sheet of niobium being interposed between the core and the matrix and serving as a diffusion barrier to prevent matrix material diffusing into the superconducting core.
The strand is obtained by successive stages of extrusion, wire drawing, and assembling a multiplicity of cut-off lengths of the element as obtained after each wire-drawing operation.
With the materials currently used, it is observed that the spacing between two superconducting filaments in a completed strand must be greater than about 0.13 .mu.m in order to satisfy above-specified conditions 2 and 3. With increasingly fine superconducting filaments, this spacing requirement reduces the superconducting fraction and thus the apparent current density.
In practice, it becomes highly penalizing to reduce the diameter of the filaments below 0.15 .mu.m, in spite of the considerable benefit that this would obtain with respect to losses and current density in the superconductor alone.
The object of the invention is to provide a super-conducting strand for use at industrial frequencies by using materials which simultaneously present metallurgical properties making them suitable for the manufacture of ultrafine filaments, and electrical properties enabling the spacing required between superconducting filaments to be considerably reduced, thereby making it possible to obtain a strand including an increased number of filaments at a smaller spacing, thus giving greatly increased apparent current densities which are multiplied by about two while losses are simultaneously divided by about two.